The move comes after the library's board announced last week that three members of the executive team will receive responsibility allowances of up to a 10 per cent pay increase, at a time when staff at the library have not had a consolidated pay rise for at least five years.

While the union has established the allowances are temporary at present it says the decision still rankles given that lower paid staff at the institution do not even receive the living wage.

Prospect negotiations executive Jane Lancaster said: "Members are aggrieved and feel that not only are management failing to treat all staff equally but there is no transparency in the decision making.

"The number of staff receiving below the living wage was a major reason why Prospect could not support the most recent pay offer tabled in February."

Lancaster said it was an issue the union will raise at the Wales TUC Conference in Llandudno this week.

Prospect will move a motion applauding the Welsh government for paying its employees the living wage, but questioning why some public bodies sponsored by the Welsh government, such as the library, do not.

"We will be calling for the Wales TUC to press the Welsh government for details of the number of public sector workers it funds who receive a salary less than the living wage.

"We will also seek a commitment to adequately fund those bodies presently paying less than the living wage, which are funded directly or indirectly by the Welsh government, so its commitment to social inclusion and economic justice is comprehensive and not selective."

Meanwhile management at the library and the unions are preparing for further talks in June to discuss an improved pay offer and recent changes to the executive team.